Using Apple's Combine Framework

import UIKit import Combine class ViewController: UIViewController { @IBOutlet weak var github_id_entry: UITextField! @IBOutlet weak var activityIndicator: UIActivityIndicatorView! @IBOutlet weak var repositoryCountLabel: UILabel! @IBOutlet weak var githubAvatarImageView: UIImageView! var repositoryCountSubscriber: AnyCancellable? var avatarViewSubscriber: AnyCancellable? var usernameSubscriber: AnyCancellable? var headingSubscriber: AnyCancellable? var apiNetworkActivitySubscriber: AnyCancellable? // username from the github_id_entry field, updated via IBAction @Published var username: String = "" // github user retrieved from the API publisher. As it's updated, it // is "wired" to update UI elements @Published private var githubUserData: [GithubAPIUser] = [] // publisher reference for this is $username, of type  var myBackgroundQueue: DispatchQueue = DispatchQueue(label: "viewControllerBackgroundQueue") let coreLocationProxy = LocationHeadingProxy() // MARK - Actions @IBAction func githubIdChanged(_ sender: UITextField) { username = sender.text ?? "" print("Set username to ", username) } // MARK - lifecycle methods override func viewDidLoad() { super.viewDidLoad() // Do any additional setup after loading the view. let apiActivitySub = GithubAPI.networkActivityPublisher (1) .receive(on: RunLoop.main) .sink { doingSomethingNow in if (doingSomethingNow) { self.activityIndicator.startAnimating() } else { self.activityIndicator.stopAnimating() } } apiNetworkActivitySubscriber = AnyCancellable(apiActivitySub) usernameSubscriber = $username (2) .throttle(for: 0.5, scheduler: myBackgroundQueue, latest: true) // ^^ scheduler myBackGroundQueue publishes resulting elements // into that queue, resulting on this processing moving off the // main runloop. .removeDuplicates() .print("username pipeline: ") // debugging output for pipeline .map { username -> AnyPublisher<[GithubAPIUser], Never> in return GithubAPI.retrieveGithubUser(username: username) } // ^^ type returned in the pipeline is a Publisher, so we use // switchToLatest to flatten the values out of that // pipeline to return down the chain, rather than returning a // publisher down the pipeline. .switchToLatest() // using a sink to get the results from the API search lets us // get not only the user, but also any errors attempting to get it. .receive(on: RunLoop.main) .assign(to: \.githubUserData, on: self) // using .assign() on the other hand (which returns an // AnyCancellable) *DOES* require a Failure type of  repositoryCountSubscriber = $githubUserData (3) .print("github user data: ") .map { userData -> String in if let firstUser = userData.first { return String(firstUser.public_repos) } return "unknown" } .receive(on: RunLoop.main) .assign(to: \.text, on: repositoryCountLabel) let avatarViewSub = $githubUserData (4) // When I first wrote this publisher pipeline, the type I was // aiming for was , where the value was an // optional. The commented out .filter below was to prevent a `nil` // GithubAPIUser object from propogating further and attempting to // invoke the dataTaskPublisher which retrieves the avatar image. // // When I updated the type to be non-optional ( // Never>) the filter expression was no longer needed, but possibly // interesting. // .filter({ possibleUser -> Bool in // possibleUser != nil // }) // .print("avatar image for user") // debugging output .map { userData -> AnyPublisher<UIImage, Never> in guard let firstUser = userData.first else { // my placeholder data being returned below is an empty // UIImage() instance, which simply clears the display. // Your use case may be better served with an explicit // placeholder image in the event of this error condition. return Just(UIImage()).eraseToAnyPublisher() } return URLSession.shared.dataTaskPublisher(for: URL(string: firstUser.avatar_url)!) // ^^ this hands back (Data, response) objects .handleEvents(receiveSubscription: { _ in DispatchQueue.main.async { self.activityIndicator.startAnimating() } }, receiveCompletion: { _ in DispatchQueue.main.async { self.activityIndicator.stopAnimating() } }, receiveCancel: { DispatchQueue.main.async { self.activityIndicator.stopAnimating() } }) .map { $0.data } // ^^ pare down to just the Data object .map { UIImage(data: $0)!} // ^^ convert Data into a UIImage with its initializer .subscribe(on: self.myBackgroundQueue) // ^^ do this work on a background Queue so we don't screw // with the UI responsiveness .catch { err in return Just(UIImage()) } // ^^ deal the failure scenario and return my "replacement" // image for when an avatar image either isn't available or // fails somewhere in the pipeline here. .eraseToAnyPublisher() // ^^ match the return type here to the return type defined // in the .map() wrapping this because otherwise the return // type would be terribly complex nested set of generics. } .switchToLatest() // ^^ Take the returned publisher that's been passed down the chain // and "subscribe it out" to the value within in, and then pass // that further down. .subscribe(on: myBackgroundQueue) // ^^ do the above processing as well on a background Queue rather // than potentially impacting the UI responsiveness .receive(on: RunLoop.main) // ^^ and then switch to receive and process the data on the main // queue since we're messin with the UI .map { image -> UIImage? in image } // ^^ this converts from the type UIImage to the type UIImage? // which is key to making it work correctly with the .assign() // operator, which must map the type *exactly* .assign(to: \.image, on: self.githubAvatarImageView) // convert the .sink to an `AnyCancellable` object that we have // referenced from the implied initializers avatarViewSubscriber = AnyCancellable(avatarViewSub) // KVO publisher of UIKit interface element let _ = repositoryCountLabel.publisher(for: \.text) (5) .sink { someValue in print("repositoryCountLabel Updated to \(String(describing: someValue))") } } }

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